Bring a large pot of salted water to a boil and cook the tortellini according to package instructions; reserve 1/2 cup of the cooking water, then drain the pasta.
Melt the butter in a large skillet over medium heat.
Add the diced onion and cook, stirring occasionally, until softened, about 3 minutes.
Add the minced garlic and cook for 1 minute, until fragrant.
Stir in the drained diced tomatoes and the squeezed thawed spinach and cook for about 3 minutes to heat through.
Add the basil, salt, pepper, and heavy cream, then simmer gently for about 5 minutes to thicken slightly.
Stir in the grated Parmesan until melted and the sauce is smooth, then remove the skillet from the heat.
Add the drained tortellini to the skillet and toss to coat; if the sauce seems too thick, loosen with some of the reserved pasta cooking water until you reach the desired consistency.